A Long Term Evolution (LTE) system is based on an orthogonal frequency division multiple access (OFDMA) technology. A time-frequency resource is divided into OFDM symbols in a time domain dimension and OFDM subcarriers in a frequency domain dimension. The smallest resource granularity is called a resource element (RE), which represents a time-frequency grid point of an OFDM symbol in the time domain and an OFDM subcarrier in the frequency domain. Service transmission in the LTE system is scheduled based on a base station. A basic time unit of scheduling is one subframe, and its typical duration is 1 millisecond. A subframe with a normal cyclic prefix includes 14 OFDM symbols, and a subframe with an extended cyclic prefix includes 12 OFDM symbols. A shorter subframe may be used in a continuously evolved LTE system or another wireless system in the future. For example, a duration of a future subframe may be equivalent to duration of one current OFDM symbol.
A specific scheduling process in the LTE system is that the base station sends a control channel. The control channel may carry scheduling information of a data channel. The control channel may be a physical downlink control channel (PDCCH) or an enhanced physical downlink control channel (EPDCCH). The data channel may be a physical downlink shared channel (PDSCH) or a physical uplink shared channel (PUSCH). The scheduling information includes control information such as resource allocation information or a modulation and coding scheme. After a User Equipment (UE) and the base station are synchronized, the UE detects the control channel in a subframe based on a subframe boundary, and receives a downlink data channel or sends an uplink data channel based on the scheduling information carried in the detected control channel. LTE further supports a carrier aggregation technology, that is, the base station configures a plurality of carriers for one UE to improve a data rate of the UE. During carrier aggregation, the plurality of carriers sent on a base station side are synchronously sent, that is, subframe boundaries of the plurality of carriers are aligned. The UE may separately receive a PDCCH and a corresponding PDSCH on each carrier. A detection process of each carrier is similar to the foregoing single carrier case. It should be noted that a carrier concept is equivalent to a serving cell concept in the LTE system. For example, accessing a carrier by the UE is equivalent to accessing a serving cell. The serving cell concept is used for description below.
A hybrid automatic repeat request (HARM) mechanism is used in the LTE system. Specifically, downlink data transmission is used as an example. The UE decodes a received PDSCH sent by a base station. If decoding is correct, for example, a cyclic redundancy check (CRC) is passed, the UE feeds back acknowledgement (ACK) information to the base station. If decoding is incorrect, for example, the CRC is not passed, the UE feeds back negative acknowledgement (NACK) information to the base station. Currently, in LTE, a subframe interval between receiving a subframe of the PDSCH by the UE and feeding back a subframe of an ACK or a NACK corresponding to the PDSCH by the UE is predefined, so as to ensure feeding back to the ACK or the NACK by the UE and receiving the ACK or the NACK by the base station.
All spectrums on which the serving cell of the current LTE system is deployed are licensed spectrums. A licensed spectrum can be used only by an operator network that purchases the licensed spectrum. More attention is paid to an unlicensed spectrum, because the unlicensed spectrum can be used by any operator. Therefore, a specific rule needs to be met to allow for coexistence of different operators. Spectrum resources will become increasingly in short supply in the future, and consequently a service capability of the LTE system based on the licensed spectrum is limited. In addition, Wi-Fi can use a large quantity of unlicensed spectrums to improve the service capability, and this brings a great competition and challenge to LTE. Therefore, using unlicensed spectrum in an LTE system is an evolution direction. An LTE system deployed on the unlicensed spectrum is called an unlicensed Long Term Evolution (U-LTE) system. Currently, a mainstream technology for U-LTE system deployment is using an U-LTE serving cell deployed on the unlicensed spectrum as a secondary serving cell and performing carrier aggregation on the secondary serving cell and a primary serving cell that is deployed on the licensed spectrum to serve the UE. In the future, the U-LTE serving cell may independently serve the UE, that is, the serving cell on the licensed spectrum is not required to provide assistance.
Considering friendly multi-system coexistence on the unlicensed spectrum, such as coexistence between the U-LTE system and a Wi-Fi system or coexistence between U-LTE systems of a plurality of operators, limitation of some coexistence rules needs to be followed when the unlicensed spectrum is used, such as a listen before talk (LBT) rule. Specifically, before sending a signal on a channel in which the U-LTE serving cell is located, the base station or the UE needs to perform clear channel assessment (CCA) on the channel in which the serving cell is located. Once it is detected that a receive power exceeds a threshold, the base station or the UE temporarily cannot send the signal on the channel. The base station or the UE can send the signal on the channel only when discovering that the channel is idle. Even in some cases, the base station or the UE needs to perform random backoff, and can send the signal on the channel only when the channel is idle during the random backoff.
In addition, several fixed TDD uplink-downlink configurations in current LTE are not used, but a flexible uplink-downlink configuration is used in the U-LTE serving cell. In this case, U-LTE cannot use a fixed uplink ACK or NACK feedback timing relationship at an interval of four subframes of FDD nor a fixed uplink ACK or NACK feedback timing relationship of each current TDD uplink-downlink configuration.
In conclusion, how to transmit an uplink ACK or NACK on a U-LTE serving cell on an unlicensed spectrum is an urgent problem to be resolved in the present invention.